Radio frequency (RF) power amplifiers are used in a variety of industrial and consumer products such as two-way radios and mobile phones. RF power amplifiers amplify a transmit signal up to a certain power level specified by regulatory authorities such as the United States Federal Communications Commission (FCC) and corresponding regulatory authorities in other countries. These regulatory authorities also specify emission limits for out-of-band signals created by, for example, unwanted harmonics generated in the RF power amplifier.
In products with transmit power limits on the order of a few Watts, the power amplifiers are typically combined with other components in a low-cost integrated circuit. However power amplifiers are susceptible to unwanted harmonics. To reduce the energy in these unwanted harmonics, external harmonic filtering networks can be used. However if the integrated circuit generates harmonics with significant energy, then these networks require large order filters, resulting in higher parts counts and higher product costs. Moreover the harmonics generated at the input of the external harmonic filtering network can self-interfere with other circuits in the chip and create other problems.
The use of the same reference symbols in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.